Liquid treatment device utilizing plasma

ABSTRACT

A liquid treatment device comprises: a first insulator including a first opening and a first inner surface, a gas being emitted into a subject liquid through the first opening; a first electrode that is at least partially disposed within a first space surrounded by the first inner surface; a second electrode that is at least partially disposed within the subject liquid; a gas supply source; and a power supply source. The first inner surface includes a first partial region which contacts the first opening. A forward end of the first electrode protrudes from the first opening to outside the first space, or the forward end retreats from the first opening into the first space by less than 3 mm. A first distance, which is a shortest distance between an outer surface of the first electrode and the first partial region, is 1 mm or greater.

BACKGROUND

1. Technical Field

The present disclosure relates to a liquid treatment device forperforming treatments of liquids by utilizing plasma.

2. Description of the Related Art

A technology for purifying or sterilizing liquids by utilizing plasma isknown. For example, Japanese Unexamined Patent Application PublicationNos. 2015-33694 and 2015-136644 disclose a liquid treatment device thatsupplies a gas into a liquid and generates plasma in the supplied gas.

SUMMARY

The present inventor has found an issue concerning the precipitation ofsilicon oxides that has not been considered in a known liquid treatmentdevice.

One non-limiting and exemplary embodiment provides a liquid treatmentdevice which is able to generate plasma more stably by reducing theprecipitation of silicon oxides.

A liquid treatment device according to an embodiment of the presentdisclosure comprises: a first insulator having a tubular shape andincluding a first opening and a first inner surface, a gas being emittedinto a subject liquid through the first opening; a first electrode thatis at least partially disposed within a first space surrounded by thefirst inner surface; a second electrode that is at least partiallydisposed within the subject liquid; a gas supply source that emits thegas into the subject liquid via the first opening by supplying the gasinto the first space; and a power supply source that generates plasma byapplying a voltage between the first and second electrodes. The firstinner surface includes a first partial region which contacts the firstopening. A forward end of the first electrode protrudes from the firstopening to outside the first space, or the forward end retreats from thefirst opening into the first space by less than 3 mm. A first distance,which is a shortest distance between an outer surface of the firstelectrode and the first partial region, is 1 mm or greater.

According to an embodiment of the disclosure, it is possible to providea liquid treatment device which is able to generate plasma more stablyby reducing the precipitation of silicon oxides.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the results of observing precipitates adhering to aninsulator as viewed from an opening of the insulator;

FIG. 2 is a graph illustrating the results of analyzing the precipitatesadhering to the insulator shown in FIG. 1;

FIG. 3 is a schematic view illustrating the configuration of a liquidtreatment device according to a first embodiment;

FIG. 4A is a sectional view illustrating an example of a first electrodein the first embodiment;

FIG. 4B is a sectional view illustrating another example of the firstelectrode in the first embodiment;

FIG. 5 is a sectional view illustrating an example of a second electrodein the first embodiment;

FIG. 6 illustrates the results of observing the first electrode and aninsulator as viewed from an opening of the insulator in the firstembodiment;

FIG. 7 is a graph illustrating the stability of plasma with respect tothe distance between the first electrode and the insulator and theamount of retreat of an end surface of the first electrode in the firstembodiment;

FIG. 8 illustrates the results of observing the first electrode and theinsulator as viewed from the opening of the insulator when the distanceis 1.1 mm and the amount of retreat is 4 mm as shown in FIG. 7;

FIG. 9 is a graph illustrating the stability of plasma with respect tothe amount of retreat of the end surface of the first electrode and theflow rate in the first embodiment;

FIG. 10A illustrates the results of observing the first electrode andthe insulator as viewed from the lateral side of the insulator when theamount of retreat is 4 mm and the flow rate is 0.3 L/min as shown inFIG. 9;

FIG. 10B illustrates the results of observing the first electrode andthe insulator as viewed from the lateral side of the insulator when theamount of retreat is 4 mm and the flow rate is 1.0 L/min as shown inFIG. 9;

FIG. 11 is a schematic view illustrating the configuration of a liquidtreatment device according to a second embodiment;

FIG. 12 illustrates the results of observing a second insulator of theliquid treatment device according to the second embodiment as viewedfrom the lateral side of the second insulator;

FIG. 13 illustrates the results of observing a first insulator of theliquid treatment device according to the second embodiment as viewedfrom the lateral side of the first insulator;

FIG. 14 is a graph illustrating the results of simulating therelationship between a second distance from the first electrode to thesecond insulator and an electric field generated on the inner surface ofthe second insulator in the second embodiment;

FIG. 15 is a schematic view illustrating the configuration of a liquidtreatment device according to a third embodiment;

FIG. 16 is a sectional view illustrating a first electrode and aninsulator in the third embodiment;

FIG. 17 is a front view illustrating the first electrode and theinsulator as viewed from the forward end thereof in the thirdembodiment;

FIG. 18 illustrates the results of simulating the position of agas-liquid interface near an insulator according to a comparativeexample of the third embodiment;

FIG. 19 illustrates the distribution of the flow velocity of a gas inthe same simulations as in FIG. 18;

FIG. 20 illustrates the results of simulating the position of agas-liquid interface near the insulator in the third embodiment;

FIG. 21 illustrates the distribution of the flow velocity of a gas inthe same simulations as in FIG. 20; and

FIG. 22 is a sectional view illustrating the vicinities of a firstelectrode and an insulator of a liquid treatment device according to amodified example of the third embodiment.

DETAILED DESCRIPTION Underlying Knowledge Forming Basis of Aspect of thePresent Disclosure

The present inventor has found that, in performing treatments of tapwater by using a known liquid treatment device, about three minutesafter discharge has started, the discharge becomes unstable, and hasalso visually checked that precipitates adhere to portions near anopening of an insulator about five minutes after the discharge hasstarted.

FIG. 1 illustrates the results of observing precipitates 90 x adheringto an insulator 50 x as viewed from the opening of the insulator 50 x.In FIG. 1, a substantially ring-like shape in white or gray having apredetermined width represents the schematic configuration of theinsulator 50 x. A first electrode 30 x is located farther backward thanthe opening of the insulator 50 x and has not been captured within theimage shown in FIG. 1. The schematic configuration of the firstelectrode 30 x that would be viewed from the opening of the insulator 50x is represented by a circle indicated by the white dashed line. FIGS. 6and 8 are also illustrated in a similar manner.

The inner diameter of the insulator 50 x is 1 mm, and the outer diameterof the first electrode 30 x is 0.8 mm. The distance between the outersurface of the first electrode 30 x and the inner surface of theinsulator 50 x, that is, a width d1 of a space 52 x is 0.1 mm. FIG. 1shows that the precipitates 90 x adhere to the inner surface of theinsulator 50 x near the opening.

FIG. 2 is a graph illustrating the results of analyzing the precipitates90 x adhering to the insulator 50 x shown in FIG. 1. More specifically,FIG. 2 shows the results of analyzing the precipitates 90 x by scanningelectron microscope (SEM) energy dispersive X-ray (EDX) spectroscopy. InFIG. 2, the horizontal axis indicates characteristic X-ray energy, andthe vertical axis indicates the strength (count value) thereof.

FIG. 2 shows that the precipitates 90 x are silicon oxide (SiO_(x))compounds. The present inventor has considered the reasons why theprecipitates 90 x adhere to the inner surface of the insulator 50 x.

The width d1 of the space 52 x between the first electrode 30 x and theinsulator 50 x in FIG. 1 is about 0.1 mm. That is, the close proximitybetween the first electrode 30 x and the insulator 50 x causesdielectric barrier discharge or creeping discharge therebetween. Thesurface of the insulator 50 x is exposed to plasma generated by such adischarge, and then, silica contained in tap water may probablyprecipitate on the inner surface of the insulator 50 x. That is, theprecipitates 90 x may be a substance formed by precipitating silicacontained in tap water.

The formation of the precipitates 90 x near the opening of the insulator50 x may cause a discharge between the precipitates 90 x and the firstelectrode 30 x. This may change the voltage used for generating adischarge between the first electrode 30 x and a second electrode (notshown). As a result, the generation of plasma may become unstable.

To address the above-described issue, according to an aspect of thepresent disclosure, there is provided a liquid treatment deviceincluding first and second electrodes, a tubular first insulator, a gassupply source, and a power supply source. The first and secondelectrodes are at least partially disposed within a subject liquid. Thetubular first insulator is disposed to surround an outer surface of thefirst electrode via a space and has an opening on an end surface whichcontacts the subject liquid. The gas supply source emits a gas into thesubject liquid via the opening by supplying the gas into the firstinsulator. The power supply source generates plasma by applying avoltage to between the first and second electrodes. A first distance,which is a distance between the outer surface of the first electrode andan inner surface of the first insulator, is 1 mm or greater.

The first electrode and the first insulator are sufficiently separatedfrom each other since the distance therebetween is 1 mm or greater.Thus, dielectric barrier discharge or creeping discharge is less likelyto be generated between the first electrode and the first insulator, andplasma is less likely to be generated therebetween accordingly. Hence,the possibility that the surface of the first insulator will be exposedto plasma is small, thereby reducing the formation of precipitates suchas silica on the inner surface of the first insulator.

The first distance may be 1 to 3 mm.

The outer surface of the first electrode and the inner surface of thefirst insulator are not excessively separated from each other since thedistance therebetween is 3 mm or smaller. This makes it easier for thesupplied gas to cover the first electrode. As a result, in the liquidtreatment device, the discharge is more stabilized, and the stablegeneration of plasma is achieved.

An end surface of the first electrode may retreat or protrude from theopening of the first insulator by 0 to 3 mm.

With this configuration, a discharge is more likely to be generatedbetween the end surface of the first electrode and a gas-liquidinterface, and the generation of a discharge between the first electrodeand the first insulator is reduced. Precipitates such as silica are lesslikely to be formed. As a result, in the liquid treatment device, thedischarge is more stabilized, and the stable generation of plasma isachieved.

The flow rate of a gas to be supplied can control the volume ofdischarge generated between the first electrode and the gas-liquidinterface.

The flow rate of the gas supplied from the gas supply source may be 0.5L/min or greater.

As the flow rate of the gas is greater, a discharge less spreads towardthe inner surface of the first insulator. Thus, the surface of the firstinsulator is less likely to be exposed to plasma, and the formation ofprecipitates such as silica is reduced. As a result, in the liquidtreatment device, the discharge is more stabilized, and the stablegeneration of plasma is achieved.

The first electrode may include an elongated cylindrical electrodeportion. The first insulator may be an elongated circular tubular memberwhich surrounds the outer surface of the electrode portion. Theelectrode portion and the first insulator may be coaxially disposed.

With this configuration, a space having a uniform width is formedbetween the electrode portion and the first insulator, and the surfaceof the first insulator is less likely to be exposed to plasma. Thevolume of the gas flowing through the space can be made uniform. As aresult, in the liquid treatment device, the stable generation of plasmais achieved.

If, while a gas is traveling within a predetermined flow channel, thewidth of the flow channel of the gas is changed, the flow velocity ofthe gas also varies locally, which may cause the occurrence of a swirl.If a swirl is generated within the first insulator, the subject liquidmay be drawn into the first insulator through its opening.

In the liquid treatment device of this aspect of the disclosure, thefirst electrode may include an elongated cylindrical electrode portionand a cylindrical support portion. The support portion is disposed atthe rear side of the electrode portion and supports the electrodeportion. The support portion is thicker than the electrode portion. Thesupport portion may include a gas supply hole which allows the gassupplied from the gas supply source to pass therethrough. An openingwidth of the gas supply hole and an inner diameter of the firstinsulator may be substantially the same.

The gas supplied from the gas supply source enters the first insulatorvia the gas supply hole, and the width of the flow channel of the gascan be made substantially uniform in a range between the gas supply holeand the first insulator. This reduces the possibility that a swirl willoccur within the first insulator, thereby making it possible to generateplasma more stably.

The liquid treatment device may further include an inlet disposed on therear side of the first insulator. The inlet is used for guiding the gassupplied from the gas supply source into the first insulator. A flowingdirection of the gas passing through the inlet may intersect with anaxial direction of the first insulator.

The gas entering through the inlet travels in a direction intersectingwith the axial direction of the first insulator and strikes the innersurface of the first insulator, and then travels within the firstinsulator in the axial direction thereof. In this manner, since thetraveling direction of the gas is changed on the inner surface of thefirst insulator, for example, the gas can flow in the axial direction ata stable velocity near the opening of the first insulator. This canreduce the possibility that a swirl of the gas will occur within thefirst insulator, thereby making it possible to generate plasma morestably.

The liquid treatment device may further include a tubular member with aclosed bottom. An opening width of the tubular member is substantiallythe same as that of the first insulator. The tubular member may beconnected to the rear side of the first insulator such that the tubularmember and the first insulator are coaxially positioned. The inlet maybe provided at a side wall of the tubular member. A flowing direction ofthe gas passing through the inlet may be substantially perpendicular tothe axial direction of the first insulator.

The gas entering through the inlet travels in a direction intersectingwith the axial direction of the tubular member and strikes the innersurface of the tubular member, for example, and then travels within thefirst insulator in the axial direction thereof. In this manner, sincethe traveling direction of the gas is changed on the side wall, forexample, the gas can flow at a stable velocity within the firstinsulator in the axial direction thereof. This can reduce thepossibility that a swirl of the gas will occur within the firstinsulator, thereby making it possible to generate plasma more stably.

When a voltage is applied to the first electrode surrounded by thetubular first insulator via a space, the Maxwell stress tensor isapplied to a gas-liquid interface (interface between the gas and asubject liquid) near the opening of the first insulator in accordancewith the strength of the electric field generated in the gas-liquidinterface. Because of this Maxwell stress tensor, the liquid maypenetrate into the first insulator via the inner surface of the firstinsulator through the opening.

The liquid treatment device of this aspect of the disclosure may furtherinclude a tubular second insulator disposed to surround the outersurface of the first electrode via a space. The second insulator may beconnected to the rear side of the first insulator such that the insideof the second insulator communicates with the inside of the firstinsulator. A second distance, which is a distance between the outersurface of the first electrode and the inner surface of the secondinsulator, may be greater than the first distance.

With this configuration, the electric field generated between the innersurface of the second insulator and the first electrode becomes smallerthan that between the inner surface of the first insulator and the firstelectrode. The subject liquid penetrated into the first insulator isless likely to enter the second insulator. As a result, in the liquidtreatment device, the stable generation of plasma is achieved.

The second distance may be determined by a value of the voltage appliedby the power supply source. The second distance may be a distance whichallows an electric field of a predetermined value or lower to begenerated on the inner surface of the second insulator by the voltageapplied by the power supply source.

Then, it is possible to form the second insulator with a suitable sizein accordance with the voltage applied between the first and secondelectrodes. That is, the size of the second insulator is not larger thannecessary, thereby implementing a smaller, lighter, less expensiveliquid treatment device.

The voltage applied to between the first and second electrodes may be 5kV or lower, and the second distance may be 2.6 mm or greater.

When the applied voltage is 5 kV or lower, the possibility that thesubject liquid will enter the second insulator can be reduced.

The voltage applied to between the first and second electrodes may be 5kV or higher, and the second distance may be 5 mm or greater.

When the applied voltage is 5 kV or higher, the possibility that thesubject liquid will enter the second insulator can be reduced.

The first electrode may include an elongated cylindrical electrodeportion. The first insulator may be a circular tubular member whichsurrounds an outer surface of a forward side of the electrode portion.The second insulator may be a tubular member which surrounds an outersurface of a rear side of the electrode portion. The electrode portionand the first and second insulators may be coaxially disposed.

The second insulator may be a circular tubular member or a squaretubular member.

With this configuration, a space having a uniform width is formedbetween the electrode portion and the first insulator and between theelectrode portion and the second insulator, thereby making the strengthof the electric field generated therebetween be uniform. It is unlikelythat the electric field in a certain portion is stronger than that inthe other portions between the electrode portion and each of the firstand second insulators, thereby reducing the possibility that the subjectliquid will enter the second insulator. As a result, in the liquidtreatment device, the stable generation of plasma is achieved.

A material for the first insulator may be different from a material forthe second insulator.

This enhances the workability of each of the first and second insulatorsand can thus precisely form each of the first and second insulators,thereby improving the reliability of the liquid treatment device.

The first and second insulators may be formed integrally together byusing the same material.

With this configuration, since fewer components are required, the weightof the liquid treatment device can be reduced. In a manufacturing methodfor the liquid treatment device, fewer steps are required for assemblingthe components, thereby reducing the cost.

According to another aspect of the present disclosure, there is provideda liquid treatment device including a first insulator, first and secondelectrodes, a gas supply source, and a power supply source. The firstinsulator has a tubular shape and includes a first opening and a firstinner surface. A gas is emitted into a subject liquid through the firstopening. The first electrode is at least partially disposed within afirst space surrounded by the first inner surface. The second electrodeis at least partially disposed within the subject liquid. The gas supplysource emits the gas into the subject liquid via the first opening bysupplying the gas into the first space. The power supply sourcegenerates plasma by applying a voltage to between the first and secondelectrodes. The first inner surface includes a first partial regionwhich contacts the first opening. A forward end of the first electrodeprotrudes from the first opening to outside the first space, or theforward end retreats from the first opening into the first space by lessthan 3 mm. A first distance, which is a shortest distance between anouter surface of the first electrode and the first partial region, is 1mm or greater.

The shortest distance between the outer surface of the first electrodeand the first partial region is 1 mm or greater. Thus, dielectricbarrier discharge or creeping discharge is less likely to be generatedbetween the first electrode and the first insulator, and plasma is lesslikely to be generated therebetween accordingly. Hence, the possibilitythat the surface of the first insulator will be exposed to plasma issmall, thereby reducing the formation of precipitates such as silica onthe inner surface of the first insulator.

In this specification, a “tubular” shape may be a “circular tubular”,“polygonal tubular”, or “funnel-like” shape. The tubular shape may berotationally symmetrical or may not be rotationally symmetrical. Theouter shape of the first insulator may be the same as the shape of thefirst space, or may be different from each other. For example, the outershape of the first insulator may be cylindrical, while the outer shapeof the first space may be a polygonal prism, and vice versa. The shapeof the first electrode may be a cylinder, a polygonal prism, or apyramid. When the shape of the first electrode, the outer shape of thefirst insulator, and the shape of the first space have central axes inthe longitudinal direction, these central axes may coincide with eachother, or may not coincide with each other.

The first opening may face downward and supply a gas downward. The firstopening may face upward and supply a gas upward. The first opening mayface in another direction and supply a gas in this direction.

The first partial region may be a ring-like region on the first innersurface within a predetermined distance (for example, 4 mm) from thefirst opening.

The forward end of the first electrode may protrude from the firstopening to outside the first space.

In the related art, by using dielectric barrier discharge or creepingdischarge as an auxiliary discharge, the voltage at which glow dischargestarts is reduced. In this aspect of the disclosure, dielectric barrierdischarge or creeping discharge is less likely to be generated. However,the forward end of the first electrode protrudes to the outside, andthus, glow discharge can start even with no discharge or a weakauxiliary discharge.

The forward end of the first electrode may protrude from the firstopening to outside the first space by 1 mm or greater.

The forward end of the first electrode may protrude from the firstopening to outside the first space by 3 mm or smaller.

This configuration makes it possible to more reliably cover the entiretyof the electrode portion protruding from the first opening with the gas,thereby generating plasma more stably.

The first inner surface may include a second partial region whichsurrounds the first electrode. The second partial region is differentfrom the first partial region. The first distance or a shortest distancebetween the outer surface of the first electrode and the second partialregion may be a distance which allows an electric field of 1.6×10⁶ V/mor lower to be generated in the first or second partial region by thevoltage applied by the power supply source.

The first distance or a shortest distance between the outer surface ofthe first electrode and the second partial region may be 2.6 mm orgreater.

The first distance or the shortest distance between the outer surface ofthe first electrode and the second partial region may be 5 mm orgreater.

The first distance may be 10 mm or smaller, which makes it possible togenerate bubbles more suitably.

A flow rate of the gas supplied from the gas supply source may be 0.5L/min or greater.

The first electrode may include an elongated cylindrical electrodeportion. The first insulator may be an elongated circular tubular memberwhich surrounds the outer surface of the electrode portion. Theelectrode portion and the first insulator may be coaxially disposed.

The first electrode may include an elongated cylindrical electrodeportion and a cylindrical support portion. The electrode portion has aforward end on a downstream side in a flowing direction of the gas and arear end on an upstream side in the flowing direction of the gas. Thesupport portion is disposed on a side of the rear end of the electrodeportion and supports the electrode portion. The support portion may bethicker than the electrode portion. The first insulator may be acircular tubular member. The support portion may include a gas supplyhole which allows the gas supplied from the gas supply source to passthrough the gas supply hole. An opening width of the gas supply hole andan inner diameter of the first insulator may be substantially the same.

The liquid treatment device may further include a pipe. The pipeincludes an inlet for guiding the gas supplied from the gas supplysource into the first space. A flowing direction of the gas passingthrough the inlet may intersect with an axial direction of the firstinsulator.

The liquid treatment device may further include a tubular member. Thetubular member has a third opening and a closed end on a side oppositethe third opening. The first insulator may have a second opening on aside opposite the first opening. A width of the third opening may besubstantially the same as a width of the second opening, and the thirdopening and the second opening may be connected to each other such thatthe tubular member and the first insulator are coaxially disposed. Theinlet may be provided at a side wall of the tubular member.

The liquid treatment device may further include a second insulator. Thesecond insulator has a tubular shape and includes a second inner surfacewhich surrounds the outer surface of the first electrode via a secondspace. The first insulator may have a second opening on a side oppositethe first opening. The second insulator may be connected to the secondopening of the first insulator such that the second space and the firstspace communicate with each other. A second distance, which is ashortest distance between the outer surface of the first electrode andthe second inner surface, may be greater than the first distance.

The outer shape of the second insulator may be the same as the shape ofthe second space, or may be different from each other. For example, theouter shape of the second insulator may be cylindrical, while the shapeof the second space may be a polygonal prism, and vice versa. When theshape of the first electrode, the outer shape of the second insulator,and the shape of the second space have central axes in the longitudinaldirection, these central axes may coincide with each other, or may notcoincide with each other.

The second distance may be a distance which allows an electric field of1.6×10⁶ V/m or lower to be generated on the second inner surface by thevoltage applied by the power supply source.

The voltage applied to between the first and second electrodes may be 5kV or lower, and the second distance may be 2.6 mm or greater.

The voltage applied to between the first and second electrodes may be 5kV or higher, and the second distance may be 5 mm or greater.

The first electrode may include an elongated cylindrical electrodeportion. The electrode portion has a forward end on a downstream side ina flowing direction of the gas and a rear end on an upstream side in theflowing direction of the gas. The first insulator may be a circulartubular member which surrounds an outer surface on a side of the forwardend of the electrode portion. The second insulator may be a tubularmember which surrounds an outer surface on a side of the rear end of theelectrode portion. The electrode portion and the first and secondinsulators may be coaxially disposed.

The second insulator may be a circular tubular member or a squaretubular member.

A material for the first insulator may be different from a material forthe second insulator.

A material for the first insulator may be the same as a material for thesecond insulator, and the first insulator may be formed integrally withthe second insulator.

Embodiments of the present disclosure will be described below in detailwith reference to the accompanying drawings.

All of the embodiments described below illustrate general or specificexamples. Numeric values, configurations, materials, components,positions and connection states of the components, steps, and the orderof steps illustrated in the following embodiments are only examples, andare not described for limiting the present disclosure. Among thecomponents illustrated in the following embodiments, the components thatare not recited in the independent claims which embody the broadestconcept of the present disclosure will be described as optionalcomponents.

In the drawings, the components are only schematically illustrated andare not necessarily illustrated precisely. The sizes and dimensionalratios of the components in the drawings are not necessarily illustratedas actual sizes and ratios. The substantially same components aredesignated by the same reference numerals, and an explanation thereofwill be given only once or simplified from the second time.

First Embodiment 1-1. Overview

An overview of a liquid treatment device 1 according to a firstembodiment will be described below with reference FIG. 3 illustratingthe configuration of the liquid treatment device 1.

As shown in FIG. 3, the liquid treatment device 1 generates plasma 4within a gas 3 emitted into a liquid 2. The gas 3 emitted into theliquid 2 remains in the liquid 2 as bubbles. The gas-liquid interface ofthe bubbles formed by the gas 3 may be closed within the liquid 2 or maycommunicate with the outer space.

The liquid 2 is a subject on which treatments are made by the liquidtreatment device 1. The liquid 2 is, for example, water such as tapwater or purified water, or an aqueous solution. As a result ofgenerating the plasma 4 within the liquid 2, the liquid treatment device1 generates active species within the liquid 2. Examples of the activespecies are hydroxyl radical (OH), hydrogen radical (H), oxygen radical(O), superoxide anion (O₂ ⁻), monovalent oxygen ion (O⁻), and hydrogenperoxide (H₂O₂).

The generated active species decompose substances contained in theliquid 2, so that the liquid treatment device 1 can sterilize the liquid2. The liquid treatment device 1 may also sterilize another liquid or agas by using the liquid 2 including active species (that is, the liquid2 subjected to plasma treatment). The plasma-treated liquid 2 may beused for various purposes including sterilizing.

1-2. Configuration

The configuration of the liquid treatment device 1 according to thefirst embodiment will be discussed below.

As shown in FIG. 3, the liquid treatment device 1 includes a treatmenttank 10, a reaction tank 15, piping 20, a first electrode 30, a firstholding portion 35, a second electrode 40, a second holding portion 45,an insulator 50, a gas supply pump 60, a liquid supply pump 70, and apower supply source 80. The individual elements forming the liquidtreatment device 1 will be discussed below in detail.

1-2-1. Treatment Tank

The treatment tank 10 is a container for storing the liquid 2. The outerconfiguration of the treatment tank 10 is any shape such as arectangular parallelepiped, a cylinder, and a sphere. The treatment tank10 may be a reservoir tank having an opening or a tray with its topopened.

The piping 20 is connected to the treatment tank 10. More specifically,the treatment tank 10 is connected to the reaction tank 15 via thepiping 20. The liquid supply pump 70 is connected to the piping 20, andthe liquid 2 circulates between the treatment tank 10 and the reactiontank 15 via the piping 20.

The treatment tank 10 is made of an acid-resistant resin material, forexample, fluoropolymers such as polytetrafluoroethylene, siliconerubber, polyvinyl chloride, stainless steel, or ceramic.

1-2-2. Reaction Tank

The reaction tank 15 is a tank within which the first and secondelectrodes 30 and 40 are disposed. More specifically, the first andsecond electrodes 30 and 40 pass through side walls of the reaction tank15. The reaction tank 15 is filled with the liquid 2. Within thereaction tank 15, the plasma 4 is generated in the gas (bubbles) 3supplied from the gas supply pump 60 as a result of generating adischarge between the first and second electrodes 30 and 40.

The outer configuration of the reaction tank 15 is any shape such as arectangular parallelepiped, a cylinder, and a sphere. The reaction tank15 may be a reservoir tank having an opening or a tray with its topopened. The reaction tank 15 may be part of the piping 20.

The reaction tank 15 is made of an acid-resistant resin material, forexample, fluoropolymers such as polytetrafluoroethylene, siliconerubber, polyvinyl chloride, stainless steel, or ceramic.

1-2-3. Piping

The piping 20 is used for forming a flow channel for the liquid 2, andis constituted by a hollow member such as a pipe, a tube, or a hose. Thepiping 20 is made of an acid-resistant resin material, for example,fluoropolymers such as polytetrafluoroethylene, silicone rubber,polyvinyl chloride, stainless steel, or ceramic.

In the first embodiment, the piping 20 connects the treatment tank 10and the liquid supply pump 70, connects the liquid supply pump 70 andthe reaction tank 15, and connects the reaction tank 15 and thetreatment tank 10. In this manner, the piping 20 connects the treatmenttank 10, the liquid supply pump 70, the reaction tank 15, and thetreatment tank 10 in this order so as to form a circulation path for theliquid 2. In FIG. 3, the solid arrows drawn along the piping 20 indicatethe direction in which the liquid 2 flows (that is, the circulatingdirection).

1-2-4. First Electrode

FIG. 4A is a sectional view illustrating an example of the firstelectrode 30 in the first embodiment. More specifically, FIG. 4Aillustrates a cross section passing through the longitudinal axis of thefirst electrode 30. As shown in FIG. 4A, the first electrode 30 includesan electrode portion 31 and a screw portion 32.

The first electrode 30 is one of a pair of electrodes for generating theplasma 4. The first electrode 30 is used as a reaction electrode aroundwhich the plasma 4 is generated.

The electrode portion 31 is an elongated cylindrical electrode portionprovided on the forward side of the first electrode 30. The diameter ofthe electrode portion 31 is the one which is sufficient to generate theplasma 4, for example, 2 mm or smaller. In this example, the diameter ofthe electrode portion 31 is 0.8 mm.

The electrode portion 31 is made of tungsten, for example, but is notrestricted thereto. The electrode portion 31 may be made of anothermetal such as aluminum, iron, or copper, or an alloy thereof.

At least part of the first electrode 30 is disposed within a space 52.More specifically, at least part of the electrode portion 31 of thefirst electrode 30 is disposed within the reaction tank 15. As shown inFIG. 4A, the electrode portion 31 is surrounded by the insulator 50 viathe space 52. When the gas 3 is not supplied to the reaction tank 15 bythe gas supply pump 60, the liquid 2 fills the space 52. At least partof the first electrode 30 is disposed within the liquid 2 and thuscontacts the liquid 2. When the gas 3 is supplied to the reaction tank15 by the gas supply pump 60, the gas 3 fills the space 52, and thus,the electrode portion 31 is surrounded by the gas 3 and does not contactthe liquid 2.

In the first embodiment, the electrode portion 31 and the insulator 50are coaxially disposed. The space 52 is formed between the electrodeportion 31 and the insulator 50, along the entire circumference of theelectrode portion 31. That is, the space 52 is a circular tubular spacehaving a substantially uniform width d1. The width d1 is a firstdistance, which is a distance between the outer surface of the firstelectrode 30 and the inner surface of the insulator 50. The width d1 is1 mm or greater. The width d1 may be 1 to 3 mm, for example.

The screw portion 32 is a metallic member supporting the electrodeportion 31. More specifically, the electrode portion 31 is pressed intothe screw portion 32 and is fixed. The screw portion 32 is electricallyconnected to the electrode portion 31 and transmits power received fromthe power supply source 80 to the electrode portion 31.

The screw portion 32 is a cylindrical support portion disposed on therear side of the first electrode 30. The diameter of the screw portion32 is greater than that of the electrode portion 31, and is 3 mm, forexample. The screw portion 32 is made of a metal which is easy to work,such as stainless steel or iron.

The screw portion 32 is supported by the first holding portion 35. Morespecifically, a male thread is formed on the outer surface of the screwportion 32 and is screwed with a female thread formed on the firstholding portion 35, thereby holding the screw portion 32 by the firstholding portion 35.

Through-holes 34 connected to the gas supply pump 60 are provided in thescrew portion 32. The through-holes 34 communicate with the space 52.Thus, the gas 3 supplied from the gas supply pump 60 is emitted into theliquid 2 stored in the reaction tank 15 from the opening 51 of theinsulator 50 via the through-holes 34 and the space 52.

In the first embodiment, the two through-holes 34 are provided in thescrew portion 32, as shown in FIG. 4A. This reduces the pressure drop ofthe gas 3 in the through-holes 34. However, one through-hole 34 or threeor more through-holes 34 may be provided.

As shown in FIG. 4A, an end surface 33 of the first electrode 30retreats from the opening 51. An amount of retreat d2 is defined by arange within which the contact between the plasma 4 generated near theend surface 33 and the inner surface of the insulator 50 can be reduced.More specifically, the amount of retreat d2 of the end surface 33 of thefirst electrode 30 is 0 to 3 mm.

The amount of retreat d2 is adjustable by axially rotating the screwportion 32. Rotating of the screw portion 32 axially shifts theelectrode portion 31 and the screw portion 32 together with respect tothe insulator 50 held by the first holding portion 35. This makes itpossible to vary the position of the end surface 33. For example, asshown in FIG. 4B, the end surface 33 of the first electrode 30 mayprotrude from the opening 51.

FIG. 4B is a sectional view illustrating another example of the firstelectrode 30 in the first embodiment. An amount of protrusion d3 isdefined by a range within which the end surface 33 can be containedwithin the gas 3 supplied from the gas supply pump 60. Morespecifically, the amount of protrusion d3 of the end surface 33 of thefirst electrode 30 is 0 to 3 mm.

1-2-5. First Holding Portion

The first holding portion 35 is a member for holding the first electrode30. In the first embodiment, the first holding portion 35 holds thefirst electrode 30 and the insulator 50 and fixes them to certainpositions of the reaction tank 15.

The female thread is formed on the first holding portion 35 and isscrewed with the male thread formed on the screw portion 32 of the firstelectrode 30. Rotating of the screw portion 32 axially can adjust theaxial position of the first electrode 30 with respect to the firstholding portion 35. The insulator 50 is fixed to the first holdingportion 35 or the reaction tank 15, and the position of the end surface33 of the first electrode 30 with respect to the opening 51 of theinsulator 50 can be adjusted. That is, the amount of retreat d2 or theamount of protrusion d3 of the end surface 33 can be adjusted.

1-2-6. Second Electrode

FIG. 5 is a sectional view illustrating an example of the secondelectrode 40 in the first embodiment. More specifically, FIG. 5illustrates a cross section passing through the longitudinal axis of thesecond electrode 40. As shown in FIG. 5, the second electrode 40includes an electrode portion 41 and a screw portion 42.

The second electrode 40 is the other one of the pair of electrodes forgenerating the plasma 4. At least part of the second electrode 40 isdisposed within the liquid 2. More specifically, the electrode portion41 of the second electrode 40 is disposed within the reaction tank 15and contacts the liquid 2.

The electrode portion 41 is an elongated cylindrical electrode portionprovided on the forward side of the second electrode 40. The size andthe material of the electrode portion 41 are the same as those of theelectrode portion 31 of the first electrode 30. However, they may bedifferent from each other.

The screw portion 42 is a metallic member supporting the electrodeportion 41. More specifically, the electrode portion 41 is pressed intothe screw portion 42 and is fixed. The screw portion 42 is electricallyconnected to the electrode portion 41 and transmits power received fromthe power supply source 80 to the electrode portion 41.

The screw portion 42 is a cylindrical support portion disposed on therear side of the second electrode portion 40. The diameter of the screwportion 42 is greater than that of the electrode portion 41, and is 3mm, for example. The screw portion 42 is made of a metal which is easyto work, such as stainless steel or iron.

The screw portion 42 is supported by the second holding portion 45. Morespecifically, a male thread is formed on the outer surface of the screwportion 42 and is screwed with a female thread formed on the secondholding portion 45, so that the screw portion 42 can be held by thesecond holding portion 45.

1-2-7. Second Holding Portion

The second holding portion 45 is a member for holding the secondelectrode 40. In the first embodiment, the second holding portion 45holds the second electrode 40 and fixes it to a certain position of thereaction tank 15.

The female thread is formed on the second holding portion 45 and isscrewed with the male thread formed on the screw portion 42 of thesecond electrode 40. This makes it possible to adjust the position ofthe electrode portion 41 positioned within the reaction tank 15.

1-2-8. Insulator (First Insulator)

The insulator 50 is an example of a first insulator disposed to surroundthe outer surface of the first electrode 30 via the space 52. Theinsulator 50 is a tubular insulator having the opening 51 formed at anend surface 53 which contacts the liquid 2. In the first embodiment, theinsulator 50 is an elongated circular tubular member which surrounds theouter surface of the electrode portion 31 of the first electrode 30.

The inner diameter of the insulator 50 is greater than the outerdiameter of the electrode portion 31. The electrode portion 31 and theinsulator 50 are coaxially disposed. The space 52 is formed in the shapeof a circular tube along the entire circumference of the electrodeportion 31, and thus prevents the electrode portion 31 from contactingthe insulator 50. The inner diameter of the insulator 50 is 3 mm, forexample, and the outer diameter of the electrode portion 31 is 0.8 mm,for example. The width d1 of the space 52 is calculated to be 1.1 mm.

The gas 3 supplied to the space 52 is emitted into the liquid 2 withinthe reaction tank 15 via the opening 51. The emitted gas 3 is diffusedinto the liquid 2 as bubbles. In this case, the opening 51 determinesthe largest size of the bubbles.

The insulator 50 is made of alumina ceramic, for example. Alternatively,the insulator 50 may be made of magnesia, zirconia, quartz, or yttriumoxide.

The shape of the insulator 50 is not restricted to a circular tube. Theshape of the insulator 50 may be a square tube. The insulator 50 is heldby the first holding portion 35, but may be fixed to the wall surface ofthe reaction tank 15. The gap between the insulator 50 and the firstholding portion 35 or the gap between the insulator 50 and the wallsurface of the reaction tank 15 may be filled with adhesive such asepoxy adhesive. This can reduce the possibility that the liquid 2 willpenetrate into the insulator 50 via the gap, which would make thedischarge unstable.

1-2-9. Gas Supply Pump

The gas supply pump 60 is an example of a gas supply source that emitsthe gas 3 into the liquid 2 via the opening 51 by supplying the gas 3into the insulator 50. The gas supply pump 60 is connected to the screwportion 32 of the first electrode 30, for example. The gas supply pump60 absorbs surrounding air, for example, as the gas 30 and supplies itto the space 52 via the through-holes 34 of the screw portion 32. Thegas 3 supplied from the gas supply pump 60 is not restricted to air. Thegas 3 may be argon, helium, nitrogen gas, or oxygen gas.

In the first embodiment, the flow rate of the gas 3 supplied from thegas supply pump 60 is 0.5 liters per minute (L/min) or greater. The gas3 supplied from the gas supply pump 60 pushes the liquid 2 out of thespace 52 via the opening 51 and covers the electrode portion 31. The gas3 is emitted into the liquid 2 within the reaction tank 15 via theopening 51.

1-2-10. Liquid Supply Pump

The liquid supply pump 70 is an example of a liquid supply unit thatcirculates the liquid 2 between the treatment tank 10 and the reactiontank 15 via the piping 20. In the first embodiment, the liquid supplypump 70 is disposed at some midpoint in the piping 20.

1-2-11. Power Supply Source

The power supply source 80 applies a voltage to between the first andsecond electrodes 30 and 40 so as to generate the plasma 4. Morespecifically, the power supply source 80 applies a pulse voltage or analternating current (AC) voltage to between the first and secondelectrodes 30 and 40.

The applied voltage is a positive-polarity high voltage pulse of 2 to 50kV/cm at a frequency of 1 Hz to 100 kHz, for example. The voltagewaveform may be any one of pulse, half-sine, and sine waves. The currentflowing between the first and second electrodes 30 and 40 is 1 mA to 3A, for example. In this example, the power supply source 80 applies apositive pulse voltage having a peak voltage of 4 kV at a frequency of30 kHz.

1-3. Operation

The operation of the liquid treatment device 1 according to the firstembodiment will be described below.

In the liquid treatment device 1, while the liquid supply pump 70 iscirculating the liquid 2, the gas supply pump 60 supplies the gas 3. Thegas 3 is supplied to the space 52 via the through-holes 34 of the screwportion 32. The liquid 2 which has filled the space 52 is emitted intothe liquid 2 within the reaction tank 15 via the opening 51. The flowrate of the gas 3 is 0.8 L/min, for example. The gas 3 fills the space52 and thus covers the electrode portion 31 of the first electrode 30.

The power supply source 80 applies a voltage, for example, a positivepulse voltage having a peak voltage of 4 kV at a frequency of 30 kHz, tobetween the first and second electrodes 30 and 40. This generates adischarge within the gas 3 (bubbles) which covers the electrode portion31 from the end surface 33 of the first electrode 30, thereby generatingthe plasma 4. Active species are then generated by the plasma 4 and areabsorbed into the liquid 2. The liquid 2 is circulating, and thus, theactive species fill the entirety of the liquid 2.

1-4. Advantages 1-4-1. Influence of Distance (Width d1) Between FirstElectrode and Insulator on Plasma

In the liquid treatment device 1 of the first embodiment, the distancebetween the outer surface of the first electrode 30 and the innersurface of the insulator 50 is 1 to 3 mm, for example.

In the liquid treatment device 1, the distance between the outer surfaceof the first electrode 30 and the inner surface of the insulator 50(that is, the width d1 of the space 52) is 1 mm or greater, and thefirst electrode 30 and the insulator 50 are sufficiently separated fromeach other. Thus, dielectric barrier discharge or creeping discharge isless likely to be generated between the first electrode 30 and theinsulator 50, and plasma is less likely to be generated therebetweenaccordingly. Hence, the possibility that the surface of the insulator 50will be exposed to plasma is small, thereby reducing the formation ofprecipitates on the inner surface of the insulator 50.

FIG. 6 illustrates the results of observing the first electrode 30 andthe insulator 50 as viewed from the opening 51 of the insulator 50. Morespecifically, FIG. 6 illustrates the results of observing the opening 51after applying a voltage to between the first and second electrodes 30and 40 for one hour by the power supply source 80 under the conditionthat 500 cc of water having a silica concentration of 72 parts permillion (ppm) was used as the liquid 2. In FIG. 6, the end surface 33 ofthe first electrode 30 is partially captured and is shown as a whiteportion near the center of the circle indicated by the dashed line.

In the liquid treatment device 1 of the first embodiment, noprecipitates are observed on the inner surface of the insulator 50 nearthe opening 51, as shown in FIG. 6. Even if a very small amount ofprecipitates adheres to the inner surface of the insulator 50, thedischarge is unlikely to become unstable since the width d1 is 1 mm orgreater.

In the liquid treatment device 1, the outer surface of the firstelectrode 30 and the inner surface of the insulator 50 are notexcessively separated from each other since the width d1 is 3 mm orsmaller. This makes it easier for the gas 3 supplied to the space 52 tocover the electrode portion 31, thereby making the configuration of thegas-liquid interface more stable. As a result, the discharge can bestabilized.

As described above, in the liquid treatment device 1 according to thefirst embodiment, the plasma 4 can be stably generated because of astabilized discharge.

1-4-2. Influence of Amount of Retreat d2 on Plasma

The relationship between the distance (width d1) from the outer surfaceof the first electrode 30 to the inner surface of the insulator 50 andthe amount of retreat d2 of the end surface 33 of the first electrode 30from the opening 51 will be discussed below.

In this example, 500 cc of water having a silica concentration of 88 ppmwas used as the liquid 2 and the power supply source 80 applied avoltage to between the first and second electrodes 30 and 40 for onehour. It was then observed whether the discharge would become unstable.The flow rate of the gas 3 supplied from the gas supply pump 60 was 1.0L/min. The inner diameter of the insulator 50 was 3 mm. Then, by varyingthe diameter of the electrode portion 31 and the position of the outerend 33 of the first electrode 30, the stability of discharge wasobserved in accordance with a combination of the width d1 and the amountof retreat d2. The results are shown in FIG. 7.

FIG. 7 illustrates the stability of the plasma 4 with respect to thedistance (width d1) between the first electrode 30 and the insulator 50and the amount of retreat d2 of the end surface 33 of the firstelectrode 30. In FIG. 7, the horizontal axis indicates the width d1, andthe vertical axis indicates the amount of retreat d2. In FIG. 7, thecircle (O) represents that the amount of precipitates of silica is verysmall and the discharge is stable, while the cross (X) represents that aconsiderable amount of precipitates is observed and the discharge isunstable. When the flickering of discharge was visually observed, thedischarge was found to be unstable. When the continuation of a certainbrightness of discharge light was observed, the discharge was found tobe stable.

FIG. 7 shows that, when the width d1 is smaller than 1 mm (morespecifically, about 0.1 mm or 0.3 mm), the discharge is not stableregardless of the amount of retreat d2. FIG. 7 also shows that, when thewidth d1 is 1.1 mm, no precipitates of silica are observed and thedischarge is stable if the amount of retreat d2 is 2 mm or 3 mm.

FIGS. 7 and 8 show that, even when the width d1 is 1.1 mm, precipitates90 x of silica are observed and the discharge is unstable if the amountof retreat d2 is 4 mm. FIG. 8 illustrates the results of observing thefirst electrode 30 and the insulator 50 as viewed from the opening 51 ofthe insulator 50 when the width d1 is 1.1 mm and the amount of retreatd2 is 4 mm as shown in FIG. 7.

Based on the above-described results, in the liquid treatment device 1according to the first embodiment, the end surface 33 of the firstelectrode 30 retreats from the opening 51 of the insulator 50 by 0 to 3mm.

Thus, a discharge is more likely to be generated between the end surface33 of the first electrode 30 and the gas-liquid interface, and thegeneration of a discharge between the first electrode 30 and theinsulator 50 is reduced. Precipitates 90 x such as silica are lesslikely to be formed. As a result, the discharge becomes more stable, andthe liquid treatment device 1 is able to generate the plasma 4 morestably.

FIG. 7 also shows that no precipitates are observed and the discharge isstable when the width d1 is 2.6 mm and the amount of retreat d2 is 3 mm.

When the end surface 33 of the first electrode 30 protrudes from theopening 51, the plasma 4 can also be generated stably. The reason forthis is that a discharge is generated mostly between the end surface 33and the gas-liquid interface, and the generation of a discharge betweenthe electrode portion 31 and the inner surface of the insulator 50 isreduced.

If the amount of protrusion d3 of the end surface 33 is 3 mm or smaller,the gas 3 is more likely to cover the entirety of the electrode portion31 including the end surface 33, thereby making the discharge morestable. More specifically, when the amount of protrusion d3 is 0 to 3mm, the discharge is stabilized and the plasma 4 can be generatedstably. If the gas 3 is able to stably cover the electrode portion 31 byincreasing the amount of gas 3, the amount of protrusion 3 d may begreater than 3 mm.

1-4-3. Influence of Flow Rate of Gas on Plasma

The relationship between the amount of retreat d2 and the flow rate(flow volume) of the gas 3 supplied from the gas supply pump 60 will bediscussed below.

Under the same conditions as those discussed with reference to FIGS. 7and 8, by changing the amount of retreat d2 and the flow rate, thestability of discharge was observed in accordance with a combination ofthe amount of retreat d2 and the flow rate. The results are shown inFIG. 9.

FIG. 9 illustrates the stability of the plasma 4 with respect to theamount of retreat d2 of the end surface 33 of the first electrode 30 andthe flow rate. In FIG. 9, the horizontal axis indicates the amount ofretreat d2, and the vertical axis indicates the flow rate. In FIG. 9,the representation of the circle (0) and that of the cross (X) aresimilar to those in FIG. 7. The distance (width d1) between the outersurface of the electrode portion 31 and the inner surface of theinsulator 50 is 1.1 mm.

FIG. 9 shows that, when the flow rate is 0.5 L/min, no precipitates arefound and the discharge is stable if the amount of retreat d2 is 1 to 3mm, while precipitates are found and the discharge is unstable if theamount of retreat d2 is 4 mm. FIG. 9 also shows that, even when theamount of retreat d2 is 4 mm, if the flow rate is 1.0 L/min, noprecipitates are found and the discharge is stable.

FIGS. 10A and 10B illustrate the results of observing the firstelectrode 30 and the insulator 50 as viewed from the lateral side of theinsulator 50 under the condition that the amount of retreat d2 is 4 mmas shown in FIG. 9. More specifically, FIG. 10A illustrates the resultswhen the flow rate is 0.3 L/min, while FIG. 10B illustrates the resultswhen the flow rate is 1.0 L/min.

In FIGS. 10A and 10B, for making it easy to observe the first electrode30 and the insulator 50 from the lateral side, the insulator 50 made oftransparent quartz is used. For easy understanding of the positionalrelationship between the first electrode 30 and the space 52, theschematic configurations of the first electrode 30 and the insulator 50are indicated by the white solid lines.

Based on a comparison between FIGS. 10A and 10B, as the flow rate isgreater, the plasma 4 follows the flow of the gas 3 more accurately andspreads less toward the insulator 50 (that is, in the horizontaldirection). In this manner, adjusting of the flow rate can control thearea where the plasma 4 is generated. More specifically, adjusting ofthe flow rate makes it possible to control the exposure of the insulator5 to the plasma 4.

In view of this point, in the liquid treatment device 1 according to thefirst embodiment, the flow rate of the gas 3 supplied from the gassupply pump 60 is 0.5 L/min or greater.

As the flow rate of the gas 3 is greater, the plasma 4 less spreadstoward the insulator 50. Thus, the surface of the insulator 50 is lesslikely to be exposed to the plasma 4, and the formation of theprecipitates 90 x such as silica is reduced. As a result, the dischargebecomes more stable, and the liquid treatment device 1 is able togenerate the plasma 4 more stably.

As the flow rate of the gas 3 is smaller, the plasma 4 spreads moretoward the insulator 50. However, if the end surface 33 of the firstelectrode 30 protrudes from the opening 51, the spread plasma 4 onlynegligibly contacts the inner surface of the insulator 50. If the endsurface 33 protrudes from the opening 51 or if the amount of retreat d2is small, the flow rate of the gas 3 may be smaller than 0.5 L/min.

Second Embodiment

A second embodiment will now be described below. In the secondembodiment, different points from the first embodiment will mainly bediscussed, and the same structure, operation, and advantages as those ofthe first embodiment may be omitted.

2-1. Configuration

FIG. 11 illustrates the configuration of a liquid treatment device 101according to the second embodiment. The liquid treatment device 101differs from the liquid treatment device 1 of the first embodiment inthat first and second insulators 150 and 155 are provided instead of theinsulator 50 and in that a first electrode 30 is fixed to the topsurface of a reaction tank 15 instead of being fixed to the bottomsurface thereof. However, the first electrode 30 may be fixed to thebottom surface or the side surface of the reaction tank 15. Similarly,the first electrode 30 in the first embodiment may be fixed to the topsurface or the side surface of the reaction tank 15.

2-1-1. First Insulator

The first insulator 150 is substantially the same as the insulator 50 inthe first embodiment but differs from the insulator 50 in that thesecond insulator 155 is connected to the rear side of the firstinsulator 150. More specifically, the first insulator 150 is a tubularinsulator which surrounds only part of the electrode portion 31 of thefirst electrode 30. For example, the first insulator 150 is a circulartubular member having an opening 151, and surrounds the outer surface ofthe forward side (front side) of the electrode portion 31 of the firstelectrode 30 via a space 152.

The first insulator 150 and the electrode portion 31 are coaxiallydisposed. A first distance D1, which is the distance between the innersurface of the first insulator 150 and the outer surface of the firstelectrode 30 (more specifically, the electrode portion 31), is the sameas the width d1 of the space 52 in the first embodiment. The firstdistance D1 is 1 mm or greater, and may be 1 to 3 mm, for example.

The first insulator 150 is fixed to the wall surface of the reactiontank 15, for example. The gap between the first insulator 150 and thewall surface of the reaction tank 15 may be filled with adhesive such asepoxy adhesive. Alternatively, the first insulator 150 may be fixed toand supported by the second insulator 155. In the second embodiment, therear side of the first insulator 150 is interconnected to the forwardside of the second insulator 155.

The material, size, and functions of the first insulator 150 are thesame as those of the insulator 50 in the first embodiment.

2-1-2. Second Insulator

The second insulator 155 is a tubular insulator which surrounds theouter surface of the first electrode 30 via a space 156. The secondinsulator 155 is interconnected to the rear side of the first insulator150 so that the inside of the second insulator 155 can communicate withthe inside of the first insulator 150. More specifically, the space 152of the first insulator 150 and the space 156 of the second insulator 155communicate with each other.

The second insulator 155 is a tubular member which surrounds the outersurface of the rear side of the electrode portion 31 of the firstelectrode 30. The rear side of the electrode portion 31 is the upstreamside in the flowing direction of a gas supplied from a gas supply pump60. The forward side of the electrode portion 31 is the downstream sidein the flowing direction of a gas. That is, the forward side of theelectrode portion 31 is the lower side of FIG. 11, while the rear sideof the electrode portion 31 is the upper side of FIG. 11. The secondinsulator 155 surrounds the rear side of the electrode portion 31, andthe first insulator 150 surrounds the forward side of the electrodeportion 31, as described above. The second insulator 155 may notnecessarily surround a screw portion 32 or the end surface of the rearside of the electrode portion 31.

As shown in FIG. 11, the distance between the inner surface of a firstportion of the second insulator 155 near the screw portion 32 and theouter surface of the first electrode 30 is narrow, while the distancebetween the inner surface of a second portion of the second insulator155 and the outer surface of the first electrode 30 is wide. A seconddistance D2, which is the shortest distance between the inner surface ofthe second portion of the second insulator 155 and the outer surface ofthe first electrode 30, is greater than the first distance D1. Thesecond distance D2 is determined by the voltage applied to between thefirst and second electrodes 30 and 40 by the power supply source 80.Details of this will be discussed later.

The shape of the second insulator 155 is a circular tube, for example,but is not restricted thereto. The shape of the second insulator 155 maybe a square tube. Alternatively, the second insulator 155 may beconfigured such that the distance between the first electrode 30 and theinner surface of the second insulator 155 progressively changes, such asin a funnel-like shape. In this case, too, the second insulator 155includes the following inner portion which surrounds the outer surfaceof the first electrode 30. The shortest distance between this innerportion and the outer surface of the first electrode 30 is greater thanthe first distance D1.

In the example in FIG. 11, the second insulator 155 is a circulartubular member, the forward side and the rear side of which are bothpartially closed. At the forward side of the second insulator 155, acircular opening having a diameter substantially the same as the outerdiameter of the first insulator 150 is provided. The first insulator 150is interconnected to this circular opening. At the rear side of thesecond insulator 155, a circular opening having a diameter substantiallythe same as the outer diameter of the screw portion 32 is provided. Thescrew portion 32 is fixed to this circular opening. A female thread tobe screwed with the male thread of the screw portion 32 may be providedon the circular opening at the rear side.

The second insulator 155 is a shallow tubular member, for example, butis not restricted thereto. The second insulator 155 may be a tubularmember elongated in the axial direction of the electrode portion 31, ormay be a hollow polyhedron, such as a hollow rectangular parallelepipedor cube.

The material for the second insulator 155 is not restricted to aspecific insulating material, and may be made of acrylic resin such aspolymethyl methacrylate (PMMA), polyphenylenesulfide (PPS),polyetheretherketone (PEEK), alumina ceramic, quartz, magnesia, orzirconia. The second insulator 155 may be made of a material differentfrom that of the first insulator 150. Alternatively, the first andsecond insulators 150 and 155 may be integrally formed by the samematerial.

2-2. Advantages

Advantages obtained by providing the second insulator 155 on the rearside of the first insulator 150 will be discussed below, together withthe underlying knowledge forming the basis of the second embodiment.

2-2-1. Underlying Knowledge Forming Basis of Second Embodiment andPrincipal Features of Second Embodiment

When a high voltage is applied to an electrode surrounded by a tubularinsulator having an opening, such as that as the first electrode 30, viaa space in order to generate plasma, the Maxwell stress tensor isapplied to the gas-liquid interface of a subject liquid near the openingof the insulator in accordance with the electric field generated in thegas-liquid interface. Because of this Maxwell stress tensor, afterconducting discharging continuously for a long time (for example, fiftyminutes or longer), the liquid may penetrate into the insulator via theinner surface of the insulator through the opening. This may make thedischarge unstable.

To address this issue, in the liquid treatment device 101 according tothe second embodiment, the forward portion of the outer surface of thefirst electrode 30 is covered with the tubular first insulator 150, andalso, the rear portion of the outer surface of the first electrode 30 iscovered with the tubular second insulator 155. The rear side of thefirst insulator 150 and the forward side of the second insulator 155 areconnected to each other.

In this case, the second distance D2 between the outer surface of thefirst electrode 30 and the inner surface of the second insulator 155 isset to be greater than the first distance D1 between the outer surfaceof the first electrode 30 and the inner surface of the first insulator150. With this configuration, the electric field generated on thesurface of the inner portion of the second insulator 155 becomes smallerthan that on the inner surface of the first insulator 150, and theMaxwell stress tensor applied to the inner surface of the secondinsulator 155 is also decreased accordingly. The liquid 2 penetratedthrough the opening 151 of the first insulator 150 is less likely toenter the second insulator 155, thereby reducing the possibility thatthe liquid 2 will reach the rear portion of the first electrode 30.

The second distance D2 is determined by the voltage applied to betweenthe first and second electrodes 30 and 40, for example. The experimentand simulation results, which will be discussed later, show that thesecond distance D2 may be set to be 2.6 mm or greater if the voltage is5 kV or lower, and the second distance D2 may be set to be 5.0 mm orgreater if the voltage is 5 kV or higher. Then, the electric fieldgenerated on the inner surface of the second insulator 155 issufficiently reduced. Setting of a suitable value of the second distanceD2 with respect to the applied voltage may contribute to considerablyreducing the penetration of the liquid 2 into the inner surface of thesecond insulator 155.

Similarly, in the liquid treatment device 1 according to the firstembodiment, the first distance (width d1) may be determined by thevoltage applied to between the first and second electrodes 30 and 40.The width d1 may be set to be 2.6 mm or greater if the voltage is 5 kVor lower, and the width d1 may be set to be 5.0 mm or greater if thevoltage is 5 kV or higher.

In the first or second embodiment, the first distance (width d1) or thesecond distance D2 may be set to be 10.0 mm or smaller. Then, bubblescan be generated more suitably.

2-2-2. Experiment Results

In the second embodiment, a positive-polarity pulse voltage having apeak voltage of 5 kV was applied to between the first and secondelectrodes 30 and 40 to generate plasma between the first electrode 30and the liquid 2. Then, it was observed whether the liquid 2 wouldpenetrate into the first and second insulators 150 and 155. As theliquid 2, tap water was used.

The first distance D1 was 1.1 mm, and the second distance D2 was 2.6 mm.Discharging was continuously conducted for two hours. The results of theexperiment are shown in FIGS. 12 and 13.

FIG. 12 illustrates the results of observing the second insulator 155 ofthe liquid treatment device 101 of the second embodiment as viewed fromthe lateral side of the second insulator 155. FIG. 13 illustrates theresults of observing the first insulator 150 of the liquid treatmentdevice 101 of the second embodiment as viewed from the lateral side ofthe first insulator 150. For making it easy to observe the inside of thefirst and second insulators 150 and 155, the first and second insulators150 and 155 made of transparent quartz are used.

The substantially rectangular region indicated by the white long dasheddotted lines at the center of FIG. 12 is a region surrounded by thesecond insulator 155, that is, the space 156. The black portionextending in the top-bottom direction at substantially the center of thespace 156 indicates the electrode portion 31 of the first electrode 30.The substantially rectangular region indicated by the white long dasheddouble-dotted lines which surrounds the electrode portion 31 (notclearly shown) in the lower side of FIG. 12 is a region surrounded bythe first insulator 150, that is, the space 152. At the upper side ofFIG. 12, the portion which surrounds the electrode portion 31 is thescrew portion 32.

In FIG. 12, the electrode portion 31 in the space 156 is clearly shown,and it is seen that the liquid 2 does not penetrate into the space 156.In contrast, in FIG. 12, the forward side of the electrode portion 31surrounded by the first insulator 150 is not clearly shown, and it isseen that the liquid is penetrated into the space 152.

The vertically extending rectangular region (circular tubular portion)indicated by the white long dashed dotted lines at the center of FIG. 13represents the first insulator 150. The black portion verticallyextending along the center of the first insulator 150 is the electrodeportion 31 of the first electrode 30. The white portion extending fromthe bottom of the electrode portion 31 downward is the plasma 4generated by discharge. The black ring-like portion extendinghorizontally at the center of FIG. 13 is another member which supportsthe first insulator 150.

In FIG. 13, the electrode portion 31 within the first insulator 150 isnot clearly shown, and it is seen that the liquid 2 is penetrated intothe first insulator 150 (the space 152 indicated by the white longdashed double-dotted lines). It was also observed that water dropletsadhered to along the inner surface of the first insulator 150, thoughthey are not easily seen in FIG. 13.

The above-described results show that the liquid 2 penetrated into thefirst insulator 150 does not reach the second insulator 155 by settingthe second distance D2 to be greater than the first distance D1. Thatis, the results show that providing of the second insulator 155 reducesthe penetration of the liquid 2 into the rear side of the electrodeportion 31.

Discharging was conducted continuously for two hours under theconditions that the first distance D1 was 2.6 mm and the second distanceD2 was changed to 4.6 mm, 7.1 mm, and 9.6 mm. After this experiment, thepenetration of the liquid 2 into the first and second insulators 150 and155 was not observed. The results show that the electric fieldsgenerated on the inner surfaces of the first and second insulators 150and 155 are sufficiently reduced when the first and second distances D1and D2 are 2.6 mm or greater.

2-2-3. Simulation Results

Results of simulating the relationship between the applied voltage andthe second distance D2 will be discussed below. The electric fieldgenerated on the inner surface of the second insulator 155 wascalculated to be 1.6×10⁶ V/m by simulations when the voltage of 5 kV wasapplied to between the first and second electrodes 30 and 40 and thesecond distance D2 was 2.6 mm. As the electric field is lower, theMaxwell stress tensor is smaller. Hence, it is estimated that, in theenvironment of an electric field of 1.6×10⁶ V/m or lower, the liquid 2will not penetrate into the second insulator 155.

FIG. 14 illustrates the results of simulating the relationship betweenthe second distance D2 from the first electrode 30 to the secondinsulator 155 and the electric field generated on the inner surface ofthe second insulator 155. More specifically, FIG. 14 illustrates theresults of simulating the electric field when the voltage applied tobetween the first and second electrodes 30 and 40 is 2.5 kV, 5 kV, and10 kV. In the graph, the electric fields generated when the seconddistance D2 is 1.1 mm, 2.6 mm, 4.9 mm, 7.1 mm, and 9.6 mm are shown.

The simulation results shown in FIG. 14 indicate how the absolute valueof the electric field generated on the inner surface of the secondinsulator 155 changes in accordance with the second distance D2. Thedashed line in FIG. 14 represents a threshold (1.6×10⁶ V/m) of theelectric field based on the above-described experiment results at whichthe liquid 2 will not probably penetrate into the second insulator 155.If the electric field is equal to or lower than the threshold, theliquid 2 is less likely to penetrate into the second insulator 155.

The simulation results in FIG. 14 show that, when the applied voltage isas high as 10 kV or lower, the electric field generated on the innersurface of the second insulator 155 can be reduced to 1.6×10⁶ V/m orlower if the second distance D2 is set to be 5 mm or greater. That is,the liquid 2 is less likely to penetrate into the second insulator 155.

When the applied voltage is as low as 5 kV or lower, the electric fieldgenerated on the inner surface of the second insulator 155 can bereduced to 1.6×10⁶ V/m or lower if the second distance D2 is set to be2.6 mm or greater.

In this manner, in the second embodiment, the second distance D2 is setin accordance with the voltage applied to between the first and secondelectrodes 30 and 40, thereby making it possible to sufficiently reducethe electric field generated on the inner surface of the secondinsulator 155. Thus, the liquid 2 is less likely to penetrate into thesecond insulator 155. As a result, the stable generation of plasma isachieved.

Third Embodiment

A third embodiment will now be described below. In the third embodiment,different points from the first embodiment will mainly be discussed, andthe same structure, operation, and advantages as those of the firstembodiment may be omitted.

3-1. Configuration

FIG. 15 illustrates the configuration of a liquid treatment device 201according to the third embodiment. As shown in FIG. 15, the liquidtreatment device 201 differs from the liquid treatment device 1 of thefirst embodiment in that a first electrode 230 and a first holdingportion 235 are provided instead of the first electrode 30 and the firstholding portion 35, respectively.

FIG. 16 is a sectional view illustrating the first electrode 230 and aninsulator 50 in the third embodiment. FIG. 17 is a front viewillustrating the first electrode 230 and the insulator 50 as viewed fromthe forward end thereof in the third embodiment. In FIG. 17, the firstholding portion 235 is not shown. For easy understanding of theschematic configurations of the individual elements in FIG. 17, thehatching used for an element in FIG. 16 is also used for the sameelement in FIG. 17.

3-1-1. First Electrode

As shown in FIGS. 16 and 17, the first electrode 230 includes anelectrode portion 231 and a screw portion 232. The first electrode 230is one of a pair of electrodes for generating plasma 4. The firstelectrode 230 is used as a reaction electrode around which the plasma 4is generated.

The electrode portion 231 is an elongated cylindrical electrode portionprovided on the forward side of the first electrode 230. The axiallength of the electrode portion 231 is the same as that of the insulator50, for example, or may be longer than that of the insulator 50. If theelectrode portion 231 is disposed such that an end surface 233 at theforward side of the electrode portion 231 retreats from the opening 51of the insulator 50, the rear side of the electrode portion 231protrudes from an opening 54 at the rear side of the insulator 50. Thelength of the electrode portion 231 is 15 to 30 mm, for example, but isnot restricted thereto.

The screw portion 232 is disposed on the rear side of the electrodeportion 231 and is an example of a cylindrical support portion whichsupports the electrode portion 231. The screw portion 232 is thickerthan the electrode portion 231. A gas supply hole 234 is formed in thescrew portion 232, as shown in FIG. 17.

The gas supply hole 234 is a hole for allowing the gas 3 supplied fromthe gas supply pump 60 to pass therethrough. The gas supply hole 234 isa through-hole which passes through the screw portion 232 in the axialdirection, for example. In the third embodiment, the opening width ofthe gas supply hole 234 and the inner diameter of the insulator 50 aresubstantially the same.

As shown in FIG. 17, the gas supply hole 234 is formed in a cross shape,as viewed from the front side. The electrode portion 231 is pressed intothe center of the cross-shaped gas supply hole 234 so that the screwportion 232 can support the electrode portion 231. The gas supply hole234 is radially formed around the electrode portion 231, as viewed fromthe front side. More specifically, the gas supply hole 234 includes twothrough-holes 234 a and 234 b. The through-holes 234 a and 234 b eachhave an elongated rectangular opening in the radial direction of thescrew portion 232 and cross at right angles at the center of the screwportion 232 as viewed from the front side. The length of each of therectangular openings of the through-holes 234 a and 234 b is the same asthe inner diameter of the insulator 50. The length of the rectangularopening is the length of the screw portion 232 in the radial direction,for example, and is the opening width of the gas supply hole 234. Thewidth of the rectangular opening is smaller than the diameter of theelectrode portion 231, for example.

The configuration of the gas supply hole 234 is not restricted to thatshown in FIG. 17. The gas supply hole 234 may include only one of thethrough-holes 234 a and 234 b or include three or more through-holes.

3-1-2. First Holding Portion

The first holding portion 235 is a member for holding the firstelectrode 230. In the third embodiment, the first holding portion 235holds the first electrode 230 and the insulator 50 and fixes them tocertain positions of the reaction tank 15.

The first holding portion 235 holds the screw portion 232, as shown inFIG. 16. More specifically, a female thread (not shown) is formed on theinner surface of the first holding portion 235 and is screwed with amale thread (not shown) formed on the outer surface of the screw portion232.

The first holding portion 235 is a tubular member having substantiallythe same inner diameter as that of the insulator 50. More specifically,as shown in FIG. 16, the first holding portion 235 has a space 236 whichcommunicates with the gas supply hole 234 and the space 52 of theinsulator 50. The inner diameter of the space 236 and that of the space52 are substantially the same. The space 236 is part of a flow channelthrough which the gas 3 supplied from the gas supply pump 60 flows. Thespace 236 serves to stabilize the flow of the gas 3 output from the gassupply hole 234 and to guide the gas 3 to the space 52 of the insulator50. The axial length of the space 236 is not particularly restricted. Asthe axial length of the space 236 is longer, the flow of the gas 3 ismore stabilized.

3-2. Advantages

Advantages obtained by setting the opening width of the gas supply hole234 to be substantially the same as the inner diameter of the insulator50 will be discussed below, together with the underlying knowledgeforming the basis of the third embodiment.

3-2-1. Underlying Knowledge Forming Basis of Third Embodiment

If a large distance (width d1 of the space 52) between the outer surfaceof the electrode portion 31 and the inner surface of the insulator 50 isset, that is, if a large inner diameter of the insulator 50 is set, thesize of the gas supply hole for guiding a gas into the insulator 50 maybecome smaller than the inner diameter of the insulator 50. In thiscase, the width of the flow channel of the gas 3 supplied through thegas supply hole becomes discontinuous at the interface between the gassupply hole and the insulator 50. Because of this discontinuity of thewidth of the flow channel, a swirl may be generated in the gas 3 nearthe inner surface of the insulator 50, and the liquid 2 captured by thisswirl may penetrate into the insulator 50 via the inner surface of theinsulator 50.

FIG. 18 illustrates the results of simulating (calculating numericvalues) the position of the gas-liquid interface near the insulator 50according to a comparative example of the third embodiment. FIG. 19illustrates the distribution of the flow velocity of the gas 3 in thesame simulations as in FIG. 18. For simple representation, in FIGS. 18and 19, a screw portion 232 x and the insulator 50 are illustrated asthin elements. FIGS. 20 and 21 are also illustrated in a similar manner.

In this comparative example, numeric values are calculated, assumingthat the opening width of a gas supply hole 234 x is 0.8 mm, the innerdiameter of the insulator 50 is 3.0 mm, the flow rate of the gas 3supplied through the gas supply hole 234 x is 1.5 L/min, and the flowrate of the liquid 2 (tap water) circulating within a reaction tank is1.0 L/min.

FIGS. 18 and 19 show that a swirl is generated near the inner surface ofthe insulator 50 and that the gas-liquid interface is drawn into theinsulator 50 through the opening 51 because of the swirl.

3-2-2. Principal Features of Third Embodiment

To address this issue, in the liquid treatment device 201 according tothe third embodiment, the opening width of the gas supply hole 234 andthe inner diameter of the insulator 50 are set to be substantially thesame, as shown in FIGS. 16 and 17. This stabilizes the distribution ofthe flow velocity of the gas 3 supplied to the inside of the insulator50 and also stabilizes the configuration of the bubbles formed at theopening 51 of the insulator 50.

FIG. 20 illustrates the results of simulating (calculating numericvalues) the position of the gas-liquid interface near the insulator 50in the third embodiment. FIG. 21 illustrates the distribution of theflow velocity of the gas 3 in the same simulations as in FIG. 20.

In this case, numeric values are calculated, assuming that the openingwidth of the gas supply hole 234 is 3.0 mm, the inner diameter of theinsulator 50 is 3.0 mm, the flow rate of the gas 3 supplied through thegas supply hole 234 is 1.5 L/min, and the flow rate of the liquid 2circulating within the reaction tank 15 is 1.0 L/min.

FIGS. 20 and 21 show that the distribution of the flow velocity of thegas 3 within the insulator 50 is uniform. FIG. 20 shows that thegas-liquid interface is positioned near the opening 51 of the insulator50 and only negligibly enters the insulator 50.

As described above, in the liquid treatment device 201 of the thirdembodiment, since the distribution of the flow velocity of the gas 3within the insulator 50 is uniform, the liquid 2 is not drawn into theinsulator 50. As a result, the stable generation of plasma is achieved.

In the third embodiment, the screw portion 232 may directly be insertedinto the insulator 50. That is, without providing the space 236, the gas3 passing through the gas supply hole 234 is directly guided into thespace 52 of the insulator 50.

3-3. Modified Examples

A modified example which makes it possible to reduce the generation of aswirl within the insulator 50 will be described below with reference toFIG. 22. FIG. 22 is a sectional view illustrating the vicinities of afirst electrode 230 and an insulator 50 in this modified example.

As shown in FIG. 22, the liquid treatment device of this modifiedexample includes a tubular portion 332 with a closed bottom and an inlet335 instead of the screw portion 232 of the first electrode 230. Theinlet 335 is, for example, a tubular opening which connects the gassupply pump 60 and the tubular portion 332.

The tubular portion 332 is a tubular member with a closed bottom havingthe same opening width as the inner diameter of the insulator 50. Thetubular portion 332 is a circular tubular member including a space 333and a circular opening 334, for example. The tubular portion 332 servesas a flowing direction changing unit that changes the travelingdirection of the gas 3 supplied from the gas supply pump 60 within thespace 333 and stably supplies the gas 3 to the insulator 50.

The tubular portion 332 is provided on the rear side of the insulator50. More specifically, the tubular portion 332 is connected to the rearside of the insulator 50 such that the tubular portion 332 and theinsulator 50 are coaxially positioned. The space 333 of the tubularportion 332 and the space 52 of the insulator 50 communicate with eachother and form the flow channel of the gas 3.

As shown in FIG. 22, the opening 334 of the tubular portion 332 isconnected to the opening 54 at the rear side of the insulator 50. Theopenings 54 and 334 have the same configuration and the same size. Thatis, the tubular portion 332 and the insulator 50 are connected to eachother so that the width of the flow channel can be formed substantiallyuniform.

The tubular portion 332 may be made of an insulating material, forexample, more specifically, acrylic resin such as PMMA, PPS, PEEK,alumina ceramic, quartz, magnesia, or zirconia.

The inlet 335 is an opening for guiding the gas 3 supplied from the gassupply pump 60 into the insulator 50. The inlet 335 is not perpendicularto the axial direction of the insulator 50. More specifically, the inlet335 is provided on the side wall of the tubular portion 332 and isdisposed substantially parallel with the axial direction of theinsulator 50. In other words, the flowing direction of the gas 3 passingthrough the inlet 335 (that is, the direction perpendicular to the inlet335) is substantially perpendicular to the axial direction of theinsulator 50. The inlet 335 may be tilted with respect to the axialdirection of the insulator 50. That is, the flowing direction of the gas3 passing through the inlet 335 may intersect with the axial directionof the insulator 50.

The gas 3 supplied into the tubular portion 332 through the inlet 335has its traveling direction changed within the space 333 of the tubularportion 332 and advances toward the insulator 50. The opening 334 of thetubular portion 332 and the opening 54 at the rear side of the insulator50 have the same configuration and the same size. Hence, without makingthe width of the flow channel of the gas 3 discontinuous (nonuniform),the gas 3 travels in the axial direction of the insulator 50 (space 52)and is emitted into the liquid 2 through the opening 51.

In this manner, without the discontinuity of the width of the flowchannel of the gas 3 within the insulator 50, the distribution of theflow velocity of the gas 3 within the insulator 50 becomes uniform. Aswirl is less likely to be generated within the insulator 50, and thus,the liquid 2 is less likely to enter the insulator 50. As a result, inthe liquid treatment device of this modified example, more stablegeneration of plasma is achieved.

In this modified example, the inlet 335 is provided on the side wall ofthe tubular portion 332 disposed at the rear side of the insulator 50.Alternatively, the inlet 335 may be provided such that it passes throughthe side wall of the insulator 50. That is, the insulator 50 may have atubular shape in which the opening 54 at the rear side is closed, andthe inlet 335 may be provided at the side wall of the insulator 50 nearthe closed opening 54. With this configuration, fewer components arerequired, thereby implementing a lighter, less expensive liquidtreatment device.

In this modified example, the insulator 50 and the tubular portion 332are directly connected to each other. However, as in the configurationshown in FIG. 16, the first holding portion 235 having a space 236 maybe provided between the insulator 50 and the tubular portion 332. Inthis case, the spaces 333, 236, and 52 have substantially the samediameter, thereby making the width of the flow channel substantiallyuniform.

OTHER EMBODIMENTS

The liquid treatment devices according to one or plural aspects havebeen described through illustration of the embodiments. However, thepresent disclosure is not restricted to the above-described embodiments.Without departing from the spirit of the present disclosure, variousmodifications apparent to practitioners skilled in the art may be madeto the embodiments, and components in the different embodiments may becombined with each other to form other aspects of the disclosure. Suchaspects are also encompassed within the scope of the disclosure.

In the above-described embodiments, the position of the end surface ofthe first electrode is adjustable. For example, in the first embodiment,rotating of the screw portion 32 adjusts the position of the end surface33 of the first electrode 30. However, the positional relationshipbetween the electrode portion 31 and the insulator 50 may be fixed. Morespecifically, a female thread may not be formed on the first holdingportion 35, and a male thread may not be formed on the screw portion 32.

In the above-described embodiments, the first electrode includes anelectrode portion and a screw portion. However, in the first embodiment,for example, the first electrode 30 may be one bar-like (cylindrical)electrode. The first electrode 30 may be a square tubular or flattenedelectrode. The second electrode 40 may be formed in a similar manner.

In the liquid treatment device 1 of the first embodiment, at least oneof the first and second holding portions 35 and 45 may be omitted, andat least one of the first and second electrodes 30 and 40 may directlybe fixed to the reaction tank 15.

In the above-described embodiments, the treatment tank 10 and thereaction tank 15 are connected to each other via the piping 20, and theliquid 2 is circulated by the liquid supply pump 70. However, in theliquid treatment device 1, for example, without providing the treatmenttank 10 and the piping 20, the plasma 4 may be generated in the stillliquid 2 (still water).

In the above-described embodiments, as the liquid 2, tap watercontaining silica is used. However, purified water or a liquidcontaining minerals, such as calcium, may be used as the liquid 2.

Various changes, replacements, addition, omission may be made to theabove-described embodiments within the spirit of the disclosure definedby the following claims and their equivalents.

The present disclosure is applicable to a liquid treatment device thatis able to generate plasma stably, for example, to a sterilizing deviceand a purifying device.

What is claimed is:
 1. A liquid treatment device, comprising: a firstinsulator having a tubular shape and including a first opening and afirst inner surface, a gas being emitted into a subject liquid throughthe first opening; a first electrode that is at least partially disposedwithin a first space surrounded by the first inner surface; a secondelectrode that is at least partially disposed within the subject liquid;a gas supply source that emits the gas into the subject liquid via thefirst opening by supplying the gas into the first space; and a powersupply source that generates plasma by applying a voltage between thefirst and second electrodes, wherein the first inner surface includes afirst partial region which contacts the first opening, a forward end ofthe first electrode protrudes from the first opening to outside thefirst space, or the forward end retreats from the first opening into thefirst space by less than 3 mm, and a first distance, which is a shortestdistance between an outer surface of the first electrode and the firstpartial region, is 1 mm or greater.
 2. The liquid treatment deviceaccording to claim 1, wherein the forward end of the first electrodeprotrudes from the first opening to outside the first space.
 3. Theliquid treatment device according to claim 2, wherein the forward end ofthe first electrode protrudes from the first opening to outside thefirst space by 1 mm or greater.
 4. The liquid treatment device accordingto claim 2, wherein the forward end of the first electrode protrudesfrom the first opening to outside the first space by 3 mm or smaller. 5.The liquid treatment device according to claim 1, wherein: the firstinner surface includes a second partial region which surrounds the firstelectrode, the second partial region being different from the firstpartial region; and the first distance or a shortest distance betweenthe outer surface of the first electrode and the second partial regionis a distance which allows an electric field of 1.6×10⁶ V/m or lower tobe generated in the first or second partial region by the voltageapplied by the power supply source.
 6. The liquid treatment deviceaccording to claim 1, wherein: the first inner surface includes a secondpartial region which surrounds the first electrode, the second partialregion being different from the first partial region; and the firstdistance or a shortest distance between the outer surface of the firstelectrode and the second partial region is 2.6 mm or greater.
 7. Theliquid treatment device according to claim 6, wherein the first distanceor the shortest distance between the outer surface of the firstelectrode and the second partial region is 5 mm or greater.
 8. Theliquid treatment device according to claim 1, wherein the first distanceis 1 to 3 mm.
 9. The liquid treatment device according to claim 1,wherein a flow rate of the gas supplied from the gas supply source is0.5 L/min or greater.
 10. The liquid treatment device according to claim1, wherein: the first electrode includes an elongated cylindricalelectrode portion; the first insulator is an elongated circular tubularmember which surrounds the outer surface of the electrode portion; andthe electrode portion and the first insulator are coaxially disposed.11. The liquid treatment device according to claim 1, the firstelectrode including an elongated cylindrical electrode portion having aforward end on a downstream side in a flowing direction of the gas and arear end on an upstream side in the flowing direction of the gas, and acylindrical support portion that is disposed on a side of the rear endof the electrode portion and supports the electrode portion, the supportportion being thicker than the electrode portion, wherein the firstinsulator is a circular tubular member, the support portion includes agas supply hole which allows the gas supplied from the gas supply sourceto pass through the gas supply hole, and an opening width of the gassupply hole and an inner diameter of the first insulator aresubstantially the same.
 12. The liquid treatment device according toclaim 1, further comprising: a pipe including an inlet for guiding thegas supplied from the gas supply source into the first space, wherein aflowing direction of the gas passing through the inlet intersects withan axial direction of the first insulator.
 13. The liquid treatmentdevice according to claim 12, further comprising: a tubular memberhaving a third opening and a closed end on a side opposite the thirdopening, wherein the first insulator has a second opening on a sideopposite the first opening, a width of the third opening issubstantially the same as a width of the second opening, and the thirdopening and the second opening are connected to each other such that thetubular member and the first insulator are coaxially disposed, and theinlet is provided at a side wall of the tubular member.
 14. The liquidtreatment device according to claim 1, further comprising: a secondinsulator having a tubular shape and including a second inner surfacewhich surrounds the outer surface of the first electrode via a secondspace, wherein the first insulator has a second opening on a sideopposite the first opening, the second insulator is connected to thesecond opening of the first insulator such that the second space and thefirst space communicate with each other, and a second distance, which isa shortest distance between the outer surface of the first electrode andthe second inner surface, is greater than the first distance.
 15. Theliquid treatment device according to claim 14, wherein the seconddistance is a distance which allows an electric field of 1.6×10⁶ V/m orlower to be generated on the second inner surface by the voltage appliedby the power supply source.
 16. The liquid treatment device according toclaim 14, wherein: the voltage applied between the first and secondelectrodes is 5 kV or lower; and the second distance is 2.6 mm orgreater.
 17. The liquid treatment device according to claim 14, wherein:the voltage applied between the first and second electrodes is 5 kV orhigher; and the second distance is 5 mm or greater.
 18. The liquidtreatment device according to claim 14, wherein: the first electrodeincludes an elongated cylindrical electrode portion having a forward endon a downstream side in a flowing direction of the gas and a rear end onan upstream side in the flowing direction of the gas; the firstinsulator is a circular tubular member which surrounds an outer surfaceon a side of the forward end of the electrode portion; the secondinsulator is a tubular member which surrounds an outer surface on a sideof the rear end of the electrode portion; and the electrode portion andthe first and second insulators are coaxially disposed.
 19. The liquidtreatment device according to claim 18, wherein the second insulator isa circular tubular member or a square tubular member.
 20. The liquidtreatment device according to claim 14, wherein a material for the firstinsulator is different from a material for the second insulator.
 21. Theliquid treatment device according to claim 14, wherein a material forthe first insulator is the same as a material for the second insulator,and the first insulator is formed integrally with the second insulator.